Active magnetic regeneration refrigeration (AMRR) uses a magnetic material both as a regenerator material (to absorb and to reject heat) and as the working material creating the thermal cycle. A regenerator is a high efficiency heat exchanger which transfers heat from the part of the refrigeration cycle going from the hot to the cold heat sink to the part going from the cold to the hot heat sink. In AMRR, the magnetic materials are both the cooling agent and the regenerative medium. The active magnetic regenerator works in the same manner for heat pump applications, where the objective is to add heat to the hot sink, instead of refrigeration's interest in removing heat from the cold sink. Magnetic refrigeration technology promises high efficiency because the magnetization and demagnetization cycle on which it is based is nearly perfectly reversible. Efficiency gains are potentially large for refrigeration just below room temperature, and even larger for refrigeration requirements at lower temperatures. Materials comprising rare earth metals, rare earth/rare earth alloys, and rare earth/transition metal compounds have the highest known cooling power densities for Active Magnetic Regenerative Refrigeration technology. The optimal rare earth materials for cooling over a specific temperature range are chosen based upon the material's magnetocaloric effect (the adiabatic temperature change upon magnetization or demagnetization), the volumetric heat capacity, fabricability, and cost considerations.
The invention is also applicable to conventional gas compression cycle based refrigeration, which may also benefit in terms of efficiency and output from the incorporation of a high specific heat passive regenerator (not a working material as in active regenerators). The same rare earth containing materials preferred for magnetic refrigeration are also preferred for a passive regenerator because the magnetic phase transition creates a very high volumetric specific heat.
Magnetic refrigerators are based on the magnetocaloric effect (MCE) of certain materials to achieve cooling. The magnetic refrigerator concept was first proposed in 1925 and successfully implemented in 1933 to reach temperatures of 0.5 K, W. F. Giauque, J. Am. Chem. Soc., 49, p. 1870, (1927), Debye, Ann. Phys. (Leipzig), 81, p. 1154, (1926), W. F. Giauque and D. P. MacDougall, Phys. Rev., 43, p. 768, (1933). In the magnetic cycle, magnetic material is magnetized in a magnetic field and placed in thermal equilibrium with a high temperature source. The material is adiabatically demagnetized, and the magnetocaloric effect reduces the material's temperature. The material is then placed in contact with a cold source and allowed to absorb heat and reach temperature equilibrium with the cold source. The material is then magnetized again and the cycle repeated. This magnetization and demagnetization can be made nearly perfectly reversible, and therefore cycles with near Carnot efficiencies are possible. As a comparison, most conventional gas cycle refrigerators are on the order of 20-30% of Carnot efficiency, C. Reid, Cryofuel Systems, Inc., private communication, (1994)., C. B. Zimm, Astronautics Technology Center, private communication, (1994), whereas a model AMRR heat pump efficiency is predicted to be approximately 63%, W. A. Steyert, J. Appl. Phys., 49, pp. 1216-1226, (1978).
Modeling has been done to show that efficient refrigeration is economically possible near room temperature when rare earth metals or rare earth containing compounds are used as the working material in a magnetic cycle, W. A. Steyert J. Appl. Phys., 49, pp. 1216-1226, (1978), G. V. Brown, J. Appl. Phys., 47, pp. 3673-3680, (1976), W. A. Steyert, J. Appl. Phys. 49, pp. 1227-1333, (1978). Unlike the very low temperature magnetic refrigerators (&lt;1.5 K) which operate in a single magnetic cycle, the cycle for refrigeration at higher temperatures can be effective at frequencies of .about.1.0 Hz. Helium gas, water, or some other suitable fluid (depending on the temperature range) can be used as a medium to transport heat from the cold to the hot heat sink. The active magnetic regenerative materials employed in such cycles are the key to implementing this technology. Only yttrium and/or rare earth containing materials have a large enough magnetocaloric effect to provide sufficient enthalpy per unit volume to make practical magnetic refrigeration possible.
The literature indicates that a given rare earth material has its strongest MCE over a temperature range of about 10-40 K centered just above the Curie temperature of the material. An example is gadolinium which has a Curie temperature of 293 K. The MCE of Gd causes an adiabatic temperature change of approximately 14 K or a isothermal heat absorption of 4 kJ/kg by the application of a 7 T magnetic field as reported by Brown, J. Appl. Phys. 47, pp. 3673-3680, (1976). Most materials will have little temperature change or heat absorption under similar conditions. This large temperature change can make magnetic refrigeration possible. For any given rare earth material, the most efficient working temperature range is restricted to approximately 40 K or less. For this reason, the highest efficiencies are realized in refrigerators covering a large temperature span when a series of rare earth containing compounds are employed. Magnetic refrigeration can be accomplished most efficiently by using different rare earth materials for different temperature ranges. A review of the literature indicates that a general consensus is forming for the preferred materials at specific temperature ranges. Some of these are summarized in Table 1 below. Choices for the optimal rare earth or alloy to use are based upon the material's magnetocaloric effect, the magnetic transition temperature, costs, and fabricability.
TABLE 1 ______________________________________ Optimal Temp. Material Range (K) ______________________________________ Gd 270-320 Gd.sub.0.6 Dy.sub.0.4 235-270 Tb 220-240 Dy 170-185 GdNi.sub.2 65-90 (Dy.sub.0.5 Er.sub.0.5)Al.sub.2 25-60 Er.sub.3 Ni 4-20 Nd 4-20 ______________________________________
The magnetic ordering near the rare earth material's Curie temperature also results in an unusually large specific heat. For example, the volumetric heat capacity of lead, neodymium, and Er.sub.3 Ni, at approximately 7 K are 0.05, 0.30 and 0.39 J/K cm.sup.3, respectively, as reported by Aprigliano, Green, Chafe, O'Connor, Biancanello, and Ridder, Adv. in Cry. Engr., vol. 37B, pp. 1003-1009, (1992). The enhancement of the heat capacity and heat absorbed or rejected due to the magnetocaloric effect is dependent on the temperature range and the material compound of interest. The enhanced heat capacity feature is advantageous for both active and passive regenerator applications.
To obtain rapid and efficient transfer of heat, the regenerator magnetic material must have a high surface area to volume ratio. The packing density of the material must also be homogeneous to allow uniform and complete heat transfer to the fluid as well as provide a uniform fluid pressure drop to minimize turbulence losses in the fluid.
Because of the above constraints, magnetic regenerator materials for AMRR's are required to be subdivided very uniformly on a size scale on the order of 50-200 .mu.m diameter (10,000-40,000 m.sup.2 /m.sup.3 surface area per unit volume) to provide adequate heat transfer at frequencies on the order of 1 Hz, C. Reid, Cryofuel Systems, Inc., private communication, (1994). The packing density should be on the order of 60% for efficient utilization of the high magnetic field volume created by a superconducting magnet.
Conventional gas cycle refrigerators working at cryogenic temperatures can also benefit from the incorporation of a magnetic regenerator, because these materials have the highest known specific heats in the temperature range near their magnetic ordering temperature. In this application, the regenerator is passive, i.e., the regenerator is only used to absorb and reject heat and is not a working material. High efficiency Gifford McMahon (GM) type cryocoolers have been already built incorporating Er.sub.3 Ni, T. Kuriyama, R. Hakamada, Y. Tokai, M. Sahashi, R. Li., O. Yoshida, K., Matsumoto, and T. Hashimoto, Adv. in Cry. Engr., Vol. 35, pp. 1261-1269, (1990) and Nd, Superconductor Week, vol. 18, no. 8, pp. 1-2, Jun. 6, (1994), as passive magnetic regenerators. The passive regenerator issues of the regenerator geometry, heat capacity, stability, fabricability, cross section uniformity, and cost are similar to the active magnetic regenerators situation. Further enhancements in the GM refrigerator performance are possible with the use of regenerator materials in fine wire form.
Many rare earth metals possess considerable ductility in pure form. Rare earth/rare earth alloys are often ductile, particularly alloys comprised of metals that are close in atomic radius. Alloys of rare earths with transition metals of interest for AMRR materials are frequently intermetallic line compounds with almost no ductility. Processes to date for manufacturing some AMRR materials have involved crushed granular materials and melt solidification of droplets.
Crushed granular materials are only applicable to intermetallic compounds which are very brittle. The process involves melting and casting of a regenerator compound of the desired composition and mechanically crushing the brittle material until it passes through an appropriate sized sieve. Material made in this fashion is not uniform in particle diameter, E. M. Ludeman and C. B. Zimm, Adv. in Cry. Engr., 37B, pp. 989-994, (1992). These particles are irregularly shaped and do not pack to a homogeneous density. As a result, the heat transfer from the regenerator material to the helium gas is less than perfect and the efficiency of a resulting regenerator is compromised.
Another means of fabricating particles of regenerator material is by forming them directly from a melt by the quenching of droplets. Centrifugal atomization processes involve the flow of liquid metal of the desired composition for a regenerator onto a disk for atomization and quenching in a bath, M. G. Osborne, I. E. Anderson, K. A. Gschneider, M. J. Gailoux, and T. W. Ellis, Adv. in Cry. Engr., 40, pp. 631-638, (1994). Low velocity jet particulation involves the solidification of liquid droplets during free fall, E. M. Ludeman and C. B. Zimm, Adv. in Cry. Engr., 37B, pp. 989-994, (1992). Several other process variations of forming particles from a melt exists, all with similar limitations. While these processes can result in powders of quite uniform spherical shape, they are limited by rather slow production rates and by reactivity of the rare earth containing material during their fabrication, as well as subsequent handling during sieving, compacting of the powders, and assembly into the regenerator.
Another problem with both the crushed granular and melt solidification type processes is the very low yield of product of an acceptable size range. It is common for the majority of the resulting powders to be too big or too small for use in a regenerator system. The out of size particles may not be recyclable due to oxygen, carbon, and nitrogen pickup. Osborne et al, M. G. Osborne, I. E., Anderson, K. A. Gschneider, M. J. Gailoux, and T. W. Ellis, Adv. in Cry. Engr., 40, pp. 631-638, (1994), report yields of 27% and 43% respectively for Nd and Er.sub.3 Ni material made by centrifugal atomization. The high cost of rare earth raw materials makes any low yield process unacceptable in terms of cost.
The fine wire form for a regenerator material provides for several potential variations in its final configuration. Theoretical modeling of regenerator materials indicate that one of the preferable configurations is in the form of wire cloth screens. Screens require the development of processes to fabricate regenerator materials in the form of fine wires. A configuration similar to a screen can also be made from expanded sheet.
The patent prior art contains several patents on magnetic refrigeration materials. U.S. Pat. No. 4,849,017 covers a compound with 50 to 75% rare earth and the remainder from one or more of the metals Co, Al, or Ni. The claims also mention sintered bodies of these compounds with densities of at least 5 g/cm.sup.3.
U.S. Pat. No. 4,985,072 covers a fine particulate morphology compound of a rare earth metal and either Al, Ni, Co, Fe in a metallic binder. The binder may be up to 80% by volume and the preferred composition mentioned is one of the elements Ag, Au, or Cu.
U.S. Pat. No. 5,213,630 and its related U.S. Pat. Nos. 5,124,215 and 4,829,770 cover magnetic refrigeration materials with compositions RE(T).sub.2 where RE is a rare earth element and T is one or more of the elements Al, Ni, Fe, or Co in a sintered body. This art describes the placing of these materials in a series of 3 or more compounds in stages to allow each stage to work at a particular temperature range and the device to cover a wide temperature range.